This invention relates generally to current transformers for providing secondary power and current sensors for monitoring electrical current, and particularly to circuit breakers employing the aforementioned current transformers and current sensors.
Current transformers (CT's) and current sensors are well known in the field of electronic circuit breakers, providing the general function of powering the electronics within the circuit breaker trip unit and sensing the circuit current within the protected circuit. While CT designs vary widely, each must address the requirements of fitting within a given volume of space, such as within a circuit breaker housing, and providing the desired level of accuracy when sensing the circuit current. A predetermined maximum core volume is required within the current transformer to ensure that the current transformer does not become magnetically saturated upon the occurrence of overcurrent conditions when used within compact circuit breakers having variable ampere ratings. Alternatively, a predetermined minimum core volume is required to insure that the core will become sufficiently magnetized at the lower steady-state operating current levels.
To provide a continuous representation of the current level within a protected circuit of an associated electrical distribution system, a current transformer is connected within the circuit breaker as described in U.S. Pat. Nos. 4,591,942 and 5,321,378 (hereinafter the '942 and '378 patents, respectively), both entitled “Current Transformer Assembly”. The current transformers as employed therein also derive operating power from the circuit current to power-up the electronic components within the circuit breaker electronic trip unit.
With regard to limiting CT size, a single iron core current transformer has been used to both sense the circuit current along with providing operational power to the electronic trip unit in higher ampere-rated circuit breakers. To prevent the iron cores from becoming saturated at higher current levels, expensive magnetic steel laminates have been used and the core size increased to allow for overload and short circuit current sensing.
In lower ampere-rated circuit breakers employing CT's for both sensing circuit current and powering up the electronic trip unit circuit, the CT size constraints require the use of expensive steel core laminations in order to optimize transformer action with the least possible amount of material and without reaching saturation.
With regard to circuit current sensing, an iron core current transformer for providing trip unit operating power and an air core current transformer for circuit current sensing have been used, as described in U.S. Pat. No. 4,297,741 (hereinafter the '741 patent) entitled “Rate Sensing Instantaneous Trip Mode Network”. However, the use of two current transformers in each pole of a circuit breaker is not always feasible because of volumetric constraints. While an improved packaging arrangement of a combination iron and air core current transformer is described in U.S. Pat. No. 5,889,450 (hereinafter the '450 patent) entitled “Current Transformer Assembly for electronic Circuit Interrupters”, the resultant specialized winding and assembly techniques result in a higher cost design.
Further with regard to circuit current sensing, a Hall-effect sensor in an air gap of a non-continuous “figure-eight” core, as described in U.S. Pat. No. 5,694,103 (hereinafter the '103 patent) entitled “Laminated FIG. 8 Power Meter Core”, has been used. The laminated figure-eight power meter core employed therein provides a ferromagnetic core for use in electric meters for converting an electrical current to a proportional magnetic flux for the detection and measurement with a Hall-effect sensor. While such an arrangement may prove feasible in power meters at lower currents or with a high volume of ferromagnetic core material, such an arrangement is still subject to the saturation considerations described above when high-currents are involved or when volumetric constraints limit the amount of ferromagnetic core material that can be used. Additionally, current sensors and power-up CT's in circuit breakers must typically operate under a broad range of current levels, such as from 0.1X to 1000X, and current sensors and CT's in overload relays must typically operate in the 0.1X–9X current range, whereas current sensors in power meters typically operate under a narrower range of current levels, such as from 0X–2X. Power meters typically require a high degree of rms-current sensing linearity in the 0X–2X range, since it is within this range that metered power usage typically operates. Overload relays typically require a high degree of rms-current sensing linearity in the 0.1X–9X range, since it is within this range that the CT must power up and the current sensor must operate for adequate overload protection. Circuit breakers typically require a high degree of rms-current sensing linearity in the 0.1X–9X range, for similar reasons as those stated for the overload relays, and a high degree of peak-current sensing capability in the 9X–1000X range, since it is within this range that the current sensor must operate for adequate short circuit protection. Root-mean-square (rms) current sensing is well known to one skilled in the art of current sensing, and generally refers to an accurate method for calculating the energy associated with a sinusoidal current wave. Peak-current sensing is also well known to one skilled in the art of current sensing, and generally refers to an accurate method for determining the occurrence of a peak current above a pre-defined threshold. Since different design considerations must be taken into account regarding circuit breaker, overload relay, and power meter applications, a ferromagnetic core that is specifically designed for the sensitivity and linearity characteristics of a power meter may not necessarily have the required sensitivity and linearity characteristics for a circuit breaker or overload relay.
The ferromagnetic cores in the aforementioned '942 and '103 patents employ a stacked-lamination core fabrication technique. An alternative fabrication technique is illustrated in U.S. Pat. No. 5,892,420 (hereinafter the '420 patent) entitled “Electronic Circuit Breaker Having Modular Current Transformer Sensors”, which produces a wound-lamination core from a continuous roll of strip metal. While both the stacked-lamination core and wound-lamination core fabrication techniques produce cores with low eddy current losses, the wound-lamination core provides an improved method for arranging separate bobbins on the core. Additionally, the wound-lamination core provides a method of fabricating a core with thinner laminations since the handling of thin laminations in a stacked-lamination core is difficult.
Magnetic cores having power-up and circuit current sensing capability may be employed in conventional circuit breakers, double break rotary circuit breakers, residential circuit breakers, commercial circuit breakers, industrial circuit breakers, air circuit breakers, overload relays, power meters, or any similar device providing electric circuit protection. Applications involving magnetic cores in circuit protective devices include, but are not limited to, the utility, industrial, commercial, residential, and automotive industries.
In view of the foregoing, it would be advantageous to provide a low cost magnetic core having a compact design, power-up capability, accurate circuit current sensing capability, and extended linearity range (dynamic range).